The present invention relates to a process for preparing high purity 2,2-bis(4-hydroxyphenyl)propane (hereinafter referred to as bisphenol A).
Bisphenol A is used as a raw material for polycarbonate resins or epoxy resins. Colorless and high purity bisphenol A is required for polycarbonate resins in particular.
Bisphenol A is prepared from acetone and excess phenol in the presence of an acidic catalyst, and in some cases by the addition of a co-catalyst such as sulfur compounds. The reaction mixture contains the catalyst, unreacted acetone, unreacted phenol, water and other by-products of the reaction.
The by-products are mainly composed of 2-(2-hydroxyphenyl)-2-(4-hydroxyphenyl) propane (hereinafter referred to as o,p' isomer). The by-products also contain Dianin's compound, trisphenol, polyphenol and undesirable colored substances. These by-products deteriorate the properties of resins prepared from bisphenol A.
Hydrochloric acid or strongly acidic ion exchange resins are known as a catalyst of this reaction. In the use of hydrochloric acid, an adduct of bisphenol A and phenol is precipitated while the reaction proceeds at a low temperature. At the same time, o,p' isomer by-products are isomerized to p,p' isomer (bisphenol A) and consequently, the amount of o,p' isomer can be reduced.
On the other hand, Dianin's compound can be reduced by adding 3% or more of water by the method described in Japanese Patent Publication 40-7186 or by the addition of mercapto-compounds as described in Japanese Patent Publication No. 27-5367. However, adding a large amount of water requires many steps, such as dehydration, and the separation and recovery of hydrochloric acid after reaction. Adding a mercapto-compound also requires complicated separation steps and results in the emission of an undesired odor. These two methods are, therefore, not practically used on an industrial scale.
Dianin's compound can also be reduced using an excess of phenol with respect to acetone. However, o,p' isomer increases with a decrease in the adduct of bisphenol-A and phenol. Furthermore, the bisphenol A must be isolated from the other substances, such as the excess phenol, present in large quantities in the reaction mixture.
Continuous reaction using an excess of phenol results in an increasing amount of the o,p' isomer. On the other hand, in a batch method, the amount of Dianin's compound increases due to the high initial concentration of acetone.
When a strongly acidic ion exchange resin is used, many impurities are produced. However, Dianin's compound is greatly reduced when a part of functional groups of the resin are modified by compounds having a mercapto-group such as mercaptoalkyl amine.
When an ion exchange resin catalyst is used, the o,p' isomer is produced to a much greater extent than when a hydrochloric acid catalyst is used because the procedure of crystallizing the adduct of phenol and bisphenol A cannot be employed.
In addition, in case of the ion exchange resin catalyst, acetone cannot be fully converted due to the water formed in the reaction as described in Japanese Patent Publication Kokai No. 61-78741. A batch method requires dehydration of the resin at each reaction, while a continuous reaction requires an enormous amount of resin for advancing the conversion to a significant extent. When acetone is isolated and recovered from water, a corrosion problem in the case of use of hydrochloric acid is not encountered. However, for isolating acetone from water and recovering the acetone, significant facilities and costs are required. Additionally, since a cation exchange resin catalyst is essentially a solid, the preparation of bisphenol A in phenol having limited solubility cannot be carried out in a high concentration, so that substantial amounts of energy and services are required for obtaining end product.
Thus, each of the conventional processes for preparing bisphenol A can reduce specific impurities, but cannot simultaneously reduce typical two impurities: o,p' isomer and Dianin's compound to a satisfactory extent.